The present invention relates in general to a method for reducing reflection from a glass surface by etching. It relates in particular to a method for applying an etchant to one surface of a sheet of glass.
It is well known that reflected glare from a glass surface may be reduced by etching the glass surface. A widely used method for etching a glass surface comprises immersing a sheet of glass in an etchant solution, usually including hydrofluoric acid. Using such a method, an etched glass sheet may be prepared in about 20 seconds.
An etched glass surface has reduced glare because light incident on the surface is reflected from the surface diffusely; that is, the light is reflected in all directions, even though it may have been incident from only one particular direction. An observer may observe the diffusely reflected light from only one direction at a time. As such, only a portion of the diffusely reflected light may be observed at a given instant. This creates the illusion of reduced reflection.
Etched glass is widely used for low-glare glazing because it is relatively inexpensive. An etched glass surface, however, may also diffuse a portion of light transmitted through the surface. As a result of the light diffusion, an observer viewing an object through an etched glass surface may be unable to resolve certain details of the object.
Immersion methods for etching a glass sheet usually provide a sheet etched on two opposite surfaces. Sheets thus produced may have limited use as cover glasses for art and photograph framing because of resolution loss due to surface diffusion. Immersion methods for etching a sheet of glass may also provide an etched surface which has a nonuniform etch. This may be due to an inability to immerse and remove a glass sheet in an etchant solution in a time period sufficiently short compared with the time required to provide an acceptable etch. A nonuniformly etched surface, however effective in reducing glare in general, may appear cosmetically unacceptable.
It is well known that a sheet of glass etched on one surface only may be as effective as a low-glare glazing for art works as a sheet etched on both surfaces. It is also well known that a one-side-etched sheet provides less distortion of art works viewed through the sheet. Immersing a sheet of glass in an etchant solution may be an unsuitable method for cost effective production of a one-side-etched sheet of glass, because one surface of the sheet must be protected during immersion in the etchant solution.
A method for modifying the light reflecting properties of one surface of glass is disclosed in U.S. Pat. No. 4,427,488, the text of which is hereby incorporated by reference. A method is disclosed whereby an etchant solution may be applied to a glass surface held in a horizontal position. It is disclosed that an etchant solution may be applied to the surface by passing it beneath a roller to which the etchant solution may be continually applied, a method generally known as roll coating. It is also disclosed that the etchant solution may be applied by passing the glass sheet below a trough containing the etchant solution. The trough has a slit in its base to allow etchant solution to flow from the trough. The glass sheet may be transferred below the slit on a series of rollers. Such a method is generally known as curtain coating. The curtain coating method applied to etching may be referred to as curtain etching.
In practice it may be very difficult to adapt a glass etching process for curtain coating, i.e. for curtain etching. Commercially available curtain coating apparatus is generally designed for use with viscous liquids, such as paints or adhesives. The apparatus is generally equipped with a pressurized liquid pumping system for forcing the viscous liquid through a slit to form a thin coating film. If leaks or breakages occur in a pressurized liquid pumping system, caustic etchant solution may be sprayed on persons or objects in the vicinity of the apparatus.
Etching solutions described in U.S. Pat. No. 4,427,488 are aqueous acid solutions and, as such, are not particularly viscous. Nonviscous liquids may form only a very thin layer on a glass sheet. Plate glass as received from a manufacturer is generally not exactly flat. It has been found that an aqueous etchant solution applied a surface of a nonflat sheet may form pools in some parts of the sheet, and in other parts may not cover the sheet sufficiently to provide a desired degree of etch. As a result, a nonflat sheet may not be uniformly etched.
Further, it is taught in U.S. Pat. No. 4,427,488 that a glass sheet should be positioned less than one millimeter below a slit delivering etchant solution to the sheet. This may also present problems in processing glass sheets which are not exactly flat, as the flatness variation of a sheet may be more than half of the gap between the slit and the sheet.
It has also been found difficult to prevent etching solution from reaching the downward facing surface of a sheet of glass. Etching solution flows over the edges of the sheet in sufficient quantity that etching occurs at least at the edges of the downward facing surface. Etching solution is also quickly transferred to rollers on which the glass sheet is transported. Etchant may thus be transferred from the rollers to the surface of the sheet resting on the rollers, i.e., the downward facing surface. Even fumes of the etching solution have caused etching on the downward facing surface of the sheet of glass.
Accordingly, it is an object of the present invention to provide a curtain etching method which may provide an etched surface on one surface of a sheet of glass.
It is another object of the invention to provide an apparatus for carrying out the curtain etching method wherein a glass sheet may be etched on one surface only while minimizing marring to the other surface due to stray etchant solution and etchant fumes.
It is a yet another object of the present invention to provide a curtain etching apparatus which does not require a pressurized pumping system.
It is still another object of the present invention to provide a curtain etching apparatus which may be used with nonflat sheets of glass
It is a further object of the present invention to provide apparatus for preventing etching solution and fumes thereof from reaching one surface of a sheet of glass.